Migrate to docusaurus

[jeluard]

Migrate existing docs (using mkdocs) to docusaurus. Find details about the rationale here.

Following items have to be done:

• [x] https://github.com/ChainSafe/lodestar/pull/6533
• [ ] https://github.com/ChainSafe/lodestar/issues/6550
• [ ] https://github.com/ChainSafe/lodestar/issues/6548 (see https://github.com/ChainSafe/lodestar/pull/6559)
• [x] https://github.com/ChainSafe/lodestar/issues/6549
• [ ] https://github.com/ChainSafe/lodestar/issues/6547
• [ ] https://github.com/ChainSafe/lodestar/issues/6575
• [x] https://github.com/ChainSafe/lodestar/pull/6566
• [x] https://github.com/ChainSafe/lodestar/pull/6560

Link redirection

Docs are currently hosted as gh-pages. If we keep this hosting, redirect will have to be handled at the website level (as opposed to HTTP level).

• https://docusaurus.io/docs/api/plugins/@docusaurus/plugin-client-redirects

Resources

• https://handbook.chainsafe.io/development/documentation/
• https://github.com/lerna/lerna/tree/main/website
• https://github.com/Consensys/doc.teku
• https://github.com/risc0/risc0/tree/main/website
• https://github.com/facebook/docusaurus/tree/main/website

[philknows]

Related #5961